1. Field of the Invention
The invention relates to a method for operating an air mass sensor arranged in the intake section of an internal combustion engine which measures the mass of the air mass flowing through the intake section. The air mass sensor has an air mass sensor element that is part of an application-specific integrated circuit constructed as a micro-electromechanical system, wherein the application-specific integrated circuit has at least one element for evaluating the measurement signals generated by the air mass sensor element.
2. Related Art
Such air mass sensors are used, for example, in motor vehicles for determining the air mass sucked in by an internal combustion engine. Combustion can be optimized by an engine controller on the basis of the most reliable possible information about a sucked-in air mass, to the effect that a quantity of fuel matched precisely to the air mass is fed into the respective combustion chambers. As result, better utilization of energy is thereby achieved with reduced emission of pollutants.
DE 44 07 209 A1 discloses an air mass sensor plugged into an intake duct in order to determine an air mass, wherein a defined portion of the total flow flows through the air mass sensor. For this purpose, the air mass sensor is embodied as a plug-in duct air mass sensor and comprises a sensor element arranged in a measurement duct, electronics for this sensor element arranged in a housing and an outlet duct on the other side of the sensor element. For a space-saving arrangement, the specified ducts or air-conducting paths are embodied in the shape of a U, S or C, forming a device having an overall compact design as a plug-in element.
Modern air mass sensor elements, which are constructed as micro-electromechanical systems (MEMS), operate very precisely and quickly. In addition, they can be manufactured cost-effectively. Unfortunately, air mass sensor elements that are designed as a micro-electromechanical system are very susceptible to contamination on the surface of the sensor element. The surface of the sensor element is in direct contact with the air flow and particles present in the air flow can damage the surface of the sensor element. This contamination problem is significantly aggravated by modern motor vehicles with an automatic start/stop system. If a hot internal combustion engine is shut down, oil droplets migrate to a greater degree from the internal combustion engine into the intake section. On the one hand, what is referred to as a chimney effect occurs, which gives rise to an air flow from the hot internal combustion engine to the cold region of the air filter in the intake section and in which, on the other hand, the thermophoresis or thermodiffusion drives the oil droplets from the hot internal combustion engine toward the cold air filter box.
The movement of particles owing to a temperature gradient within a fluid is referred to as thermophoresis, thermodiffusion or the Ludwig-Soret effect. In most cases, the movement occurs from hot to cold, but a movement toward the hotter region is also possible depending on the type of particles and the fluid. Thermophoresis occurs in all materials, and this effect can be clearly observed in the case of aerosols, such as oil droplets in the air, and also in the case of dust particles in air.
Without temperature gradient, air molecules impact on average uniformly from all sides against a dust particle or an oil droplet in air. As a result, the dust particle or oil droplet moves according to Brown's Laws and its movement is statistical and nondirectional and on average the dust particle or oil droplet does not move away from its location over a large number of Brownian impacts. If the dust particle or the oil droplet is, however, located in a temperature gradient, molecules impact faster on the hot side than on the cold side, and the particle therefore experiences a net impulse in the direction of the cold side. The movement is still statistical, but the dust particle or oil droplet moves on average for a longer time in the direction of the cold region of the air.